This application is based on Japanese Patent Application No. HEI 11-280866 filed on Sep. 30, 1999 in Japan, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention pertains to a liquid crystal composition and a liquid crystal light modulating element using the liquid crystal composition, such as a liquid crystal display.
2. Description of Related Art
In recent years, various types of reflective liquid crystal displays that use chiral nematic liquid crystal that exhibits a cholesteric phase at room temperature and is created by adding a chiral agent to nematic liquid crystal have been studied. These elements perform display by switching the liquid crystal between a planar state (colored) and a focal conic state (transparent) based on the impression of pulse signals having different levels of energy. These elements may also be constructed such that they maintain their colored or transparent state even after the application of voltage is stopped.
It is anticipated that this type of liquid crystal may be used in areas other than the display of moving images in monitors. A representative use is as a replacement for paper on which information such as images and text is printed, such as an electronic blackboard or billboard, an electronic book, or a decoration panel.
However, currently used reflective liquid crystal displays using chiral nematic liquid crystal have an insufficient Y-value (luminous reflectance) and therefore low contrast. Displays having sufficient color purity characteristics do not exist. Moreover, it is important to increase the temperature compensation range for this type of liquid crystal display.
The principal object of the present invention is to provide an improved chiral nematic liquid crystal composition and liquid crystal light modulating element.
Another object of the present invention is to provide a chiral nematic liquid crystal composition and liquid crystal light modulating element that offer superior bidirectional stability, good color purity and reflectance, high contrast, a wide temperature compensation range and a low drive voltage.
To achieve at least one of the above described objects, a chiral nematic liquid crystal composition according to the present invention comprises: a nematic liquid crystal composition including at least one kind of liquid crystal ester compound, at least one kind of liquid crystal terphenyl compound, and at least one kind of liquid crystal tolane compound; and at least one chiral agent, wherein the chiral nematic liquid crystal composition exhibits a cholesteric phase at room temperature.
A chiral nematic liquid crystal composition exhibiting a cholesteric phase at room temperature, is switched between a selective reflection state, in which the chiral nematic liquid crystal composition selectively reflects light having a specified peak wavelength, and a transparent state, in which it allows all incident light to pass through, by impressing pulse voltages having different energy amounts to the chiral nematic liquid crystal composition.
A liquid crystal ester compound has a large dielectric anisotropy, which increases responsiveness to drive voltage and increases contrast when included in the liquid crystal composition. In particular, it is desirable that the compound includes an F group or a CN group, which would improve the transparency of the chiral nematic liquid crystal composition, allow the drive voltage to be reduced, and increase the range of operating temperatures.
A liquid crystal terphenyl compound has a large refractive anisotropy, which increases scattering, thereby increasing the contrast and increasing the operating temperature range when included in the chiral nematic liquid crystal composition. It also helps to maintain the long-term reliability and stability of the liquid crystal. It is particularly desirable that the compound includes a polar group such as an F group or a Cl group, which would allow the drive voltage to be reduced.
A liquid crystal tolane compound has the effect of reducing viscosity, and including it in the liquid crystal composition can reduce the viscosity of the liquid crystal composition while maintaining high reflectance.
In the chiral nematic liquid crystal composition pertaining to the present invention, the total amount of liquid crystal ester compound and liquid crystal terphenyl compound is preferably at least 50% by weight of the amount of nematic liquid crystal composition. In particular, it is desirable for the amount of liquid crystal terphenyl compound to comprise at least 10% by weight of the amount of nematic liquid crystal composition. Furthermore, it is desirable for the amount of liquid crystal ester compound to comprise at least 30% by weight of the amount of nematic liquid crystal composition.
The nematic liquid crystal composition may also contain at least one type of liquid crystal phenyl cyclohexyl compound. A liquid crystal phenyl cyclohexyl compound contributes to reducing the viscosity and increasing the reflectance of the liquid crystal composition.
It is desirable for the amount of chiral agent contained in the chiral nematic liquid crystal composition to be at least 10% to 45% by weight of the total amount of chiral nematic liquid crystal composition. Maintaining the amount of chiral agent within this range allows the selective reflection wavelength to be set within the desired range without causing crystal separation.
It is furthermore desirable for the nematic liquid crystal composition to have a dielectric anisotropy value between 10 and 30, and ideally between 15 and 30, as well as a refractive anisotropy value between 0.1 and 0.3, and ideally between 0.15 and 0.25.
Pigments or UV-absorbent substances may also be added to the chiral nematic liquid crystal composition. Adding pigments allows the color purity of the chiral nematic liquid crystal composition to be adjusted. Adding UV-absorbent substances prevents deterioration of the chiral nematic liquid crystal composition caused by exposure to ultraviolet light.
According to another aspect of the present invention, a liquid crystal light control device comprises: a liquid crystal light control layer including a chiral nematic liquid crystal composition exhibiting a cholesteric phase at room temperature, the chiral nematic liquid crystal composition comprising: a nematic liquid crystal composition including a liquid crystal ester compound, a liquid crystal terphenyl compound, and a liquid crystal tolane compound; and at least one chiral agent.
This type of liquid crystal light modulating element (i) offers high-contrast display with good color purity and reflectance, (ii) may be driven with good responsiveness at low voltage, and (iii) has a wide temperature compensation range.